Protective garments are designed to protect the wearer from hazardous environmental conditions the wearer might encounter. Such garments include those designed to be worn by firefighters and other rescue personnel, industrial and electrical workers, and military personnel.
Standards have been promulgated that govern the performance of such garments (or constituent layers or parts of such garments) to ensure that the garments sufficiently protect the wearer in hazardous situations. For example, NFPA 2112 (2012 edition, incorporated herein by this reference) governs the required performance of industrial worker garments that protect against flash fires. National Fire Protection Association 1971: Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting (2013 edition, incorporated herein by this reference) (hereinafter “NFPA 1971”) governs the required performance of firefighter garments.
Structural firefighter garments, such as firefighters' turnout gear, typically consist of matching coat and pants and are designed primarily to prevent the wearer from sustaining a serious burn. NFPA 1971 compliant turnout gear or garments 10 are typically comprised of three layers (as shown in FIGS. 1 and 2): an outer shell 12, an intermediate moisture barrier 14, and a thermal liner 20. The outer shell 12 is usually a woven fabric made from flame resistant fibers and is considered a firefighter's first line of defense. Not only should it resist flame, but it needs to be tough and durable so as not to be torn, abraded, or snagged during normal firefighting activities.
The moisture barrier 14, which is also flame resistant, is present to keep water, harmful chemicals, bacteria, and bodily fluids from penetrating the turnout gear and affecting the wearer. The moisture barrier 14 can be constructed of a non-woven or woven flame resistant fabric 16 that is laminated to a water-impermeable layer of material 18 such as, for instance, a layer of expanded polytetrafluoroethylene (“ePTFE”), polyurethane, or combinations thereof.
The thermal liner 20 is flame resistant and offers the bulk of the thermal protection afforded by the ensemble. A traditional thermal liner consists of a batting of flame resistant thermal insulating materials 22 quilted to a lightweight facecloth 24, also made of flame resistant fibers. The facecloth 24 is commonly quilted to the batting 22 in a cross-over or chicken wire pattern. The thermal liner 20 is the innermost layer of the firefighter's garment, with the facecloth 24 typically facing the wearer.
The batting 22 of the thermal liner can be a single layer of nonwoven fabric, but more typically is formed of multiple nonwoven layers. For example, the nonwoven fabrics used in many thermal liners are 1.5 ounces per square yards (“osy”) and/or 2.3 osy spunlace fabrics. Regardless of whether single or multiple layers are used, conventional battings are designed to be relatively bulky as such a property has been thought necessary to trap air and thereby impart the required thermal protection to the wearer.
The thermal protection that a garment fabric affords the wearer is measured by determining the fabric's Thermal Protective Performance (TPP) in accordance with ISO 17492: Clothing for protection against heat and flame—Determination of heat transmission on exposure to both flame and radiant heat (2003, incorporated herein by this reference), as modified by NFPA 1971. The TPP test predicts the rate at which radiant and convective heat transfer through the three layers of the garment fabric (outer shell, moisture barrier, and thermal liner) to a level that will cause a second-degree burn to the human skin. More specifically, the test measures the amount of time at a given energy level it takes for enough heat to pass through the composite to cause a second degree burn. The minimum TPP rating for NFPA 1971-compliant coats and trousers is 35 calories/cm2 (which equates to about 17.5 seconds of protection before a second-degree burn results). The higher the number, the more protective the garment system is considered. The TPP test method is fully described in chapter 8.10 of NFPA 1971.
While TPP is a measure of the ability of the garment fabric to protect the wearer from heat and flame, it must be balanced with the Total Heat Loss (THL) of the fabric. THL measures the ability of the garment fabric to allow heat and moisture vapor to escape from the wearer through the fabric to thereby avoid heat stress on the wearer. Typically there is a tradeoff between TPP and THL—the performance of one must be sacrificed to improve the performance of the other.
The testing methodology used for measuring THL is set forth in ASTM F 1868-14: Standard Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate (2002, incorporated herein by this reference), as modified by NFPA 1971. Generally, however, the garment fabric (consisting of the outer shell, moisture barrier, and thermal liner) is laid on a 35° C. (+/−0.5° C.) hot plate in an environment with an air temperature of 25° C. (+/−0.5° C.). The test is conducted with both a wet and a dry hot plate. The amount of energy (measured in watts/m2) it takes to maintain the hot plate at 98.6° F. is measured. Higher THL values mean that more energy must be supplied to the plate to maintain the temperature because the fabric is permitting heat to escape through the garment fabric. Thus, the higher the THL value, the less insulative the fabric but the less risk of the fabric contributing the heat stress of the wearer. A minimum THL value of 205 watts/m2 is required to comply with NFPA 1971.